Assigning to vs. from a slice

Question

When reading profile.py of python standard library I came across the assignment statement sys.argv[:] = args, which is used to modify sys.argv to make the program being profiled see the correct command line arguments. I understand that this is different from sys.argv = args[:] in the actual operations, but in effect they look the same to me. Is there a situation when one wants to use one and not the other? And is a[:] = b a common python idiom?

UPDATE: in this specific situation why would one choose one over the other? (source can be found in the main function of profile.py)

Answer 1

The difference is, when you use a[:] = b it means you will override whatever is already on a. If you have something else with a reference to a it will change as well, as it keeps referencing the same location.

In the other hand, a = b[:] creates a new reference and copy all the values from b to this new reference. So existing references to the old data will keep pointing to the old data.

Consider this example:

>>> a = [1, 2, 3]
>>> b = [4, 5, 6]
>>> c = a # c is a reference to the list in a
>>> c
[1, 2, 3]
>>> 
>>> a[:] = b
>>> a # a will have a copy of the list in b
[4, 5, 6] 
>>> c # and c will keep having the same value as a
[4, 5, 6]
>>>
>>> b = [7, 8, 9]
>>> a = b[:]
>>> a # a has the new value
[7, 8, 9]
>>> c # c keeps having the old value
[4, 5, 6]